Method and circuit for limiting a pumped voltage

ABSTRACT

A method and circuit control the value of generated voltage derived from a supply voltage as the value of the supply voltage varies, such as during burn-in of an integrated circuit. A voltage generation circuit includes a generator circuit that receives a supply voltage and has a reference node and develops an output voltage from the supply voltage, the output voltage having a value that is a function of a reference voltage applied on the reference node. A coupling circuit receives the supply voltage and operates in response to a voltage control signal to vary an electronic coupling of the supply voltage to the reference node to thereby adjust the value of the reference voltage. A voltage sensing circuit develops the voltage control signal that is applied to the coupling circuit in response to the reference voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/267,225, filed Oct. 8, 2002 now U.S. Pat. No. 6,750,639, which is adivisional of U.S. patent application Ser. No. 09/996,452, filed Nov.28, 2001, issued Feb. 10, 2004 as U.S. Pat. No. 6,690,148.

TECHNICAL FIELD

The present invention relates generally to voltage generation circuits,and, more particularly, to controlling the voltage developed by avoltage generation circuit.

BACKGROUND OF THE INVENTION

Voltage generation circuits are utilized in many integrated circuits togenerate voltages required for proper operation of the integratedcircuit. For example, in a semiconductor memory device such as a dynamicrandom access memory (DRAM) a supply voltage VCC is applied to thedevice and a voltage generation circuit within the memory devicegenerates a pumped voltage VCCP having a value greater than the supplyvoltage. In a DRAM, the pumped voltage VCCP is utilized, for example, indriving word lines of a memory-cell array when accessing rows of memorycells contained in the array, as will be appreciated by those skilled inthe art. The value of the pumped voltage VCCP is greater than the supplyvoltage VCC so that capacitors in the memory cells may be charged to thesupply voltage, as will once again be understood by those skilled in theart.

FIG. 1 is a functional block diagram and schematic illustrating aconventional voltage generation circuit 100 that may be utilized in aDRAM to generate a pumped voltage VCCP having a value greater than anapplied supply voltage VCC. The voltage generation circuit 100 includesan oscillator 102 that generates an oscillator signal OSC in response toan enable signal EN applied by a Schmitt Trigger comparator 104. Theoscillator 102 clocks the OSC signal when the EN signal is active anddoes not clock the OSC signal when the EN signal is inactive, insteadmaintaining the OSC signal either high or low. The OSC signal is appliedto clock a charge pump circuit 106 which, in response to the OSC signal,generates the pumped voltage VCCP. More specifically, when the OSCsignal clocks the charge pump circuit 106, the circuit turns ON andcharges a load capacitor 108 to thereby develop the pumped voltage VCCPand drive a load resistance 109. When the OSC signal does not clock thecharge pump circuit 106, the circuit turns OFF and stops charging theload capacitor 108. The detailed operation and circuitry for forming theoscillator 102 and charge pump circuit 106 are well understood by thoseskilled in the art, and thus, for the sake of brevity, these componentswill not be described in further detail.

The pumped voltage VCCP is applied through a diode-coupled PMOStransistor 110 and a level shifting circuit 112 to develop a pumpfeedback voltage VPF that is applied to a first input of the SchmittTrigger comparator 104. The diode-coupled transistor 110 functions as alevel shifter to reduce the value of the pumped voltage VCCP and ensureproper common-mode operation of the Schmitt Trigger comparator 104, aswill be appreciated by those skilled in the art. The level shiftingcircuit 112 reduces the voltage from the diode-coupled transistor 110 byan offset voltage VOFF, which has a value determined, in part, by thedesired value of the pump feedback voltage VPF. A current source 114causes a desired current to flow through the diode-coupled transistor110 and level shifting circuit 112 so that the feedback voltage VPFhaving the desired value is developed on the first input of the SchmittTrigger comparator 104. A second input of the Schmitt Trigger comparator104 receives a reference voltage VREF that is developed by adiode-coupled PMOS transistor 116 and a current source 118 coupled inseries between the supply voltage VCC and ground. The diode-coupledtransistor 116 functions as a level shifter to reduce the value of thesupply voltage VCC and provide for proper common mode operation of theSchmitt Trigger comparator 104, as will be appreciated by those skilledin the art. The current source 118 causes a desired current to flowthrough the diode-coupled transistor 116 to develop the referencevoltage VREF on the second input of the Schmitt Trigger comparator 104.

The voltage generation circuit 100 further includes over voltageprotection components that attempt to limit the value of the pumpedvoltage VCCP as the supply voltage VCC increases. The overvoltageprotection components include an overvoltage detector 120 that monitorsthe supply voltage VCC and develops an overvoltage signal OV having avalue that is a function of the monitored supply voltage. Theovervoltage signal OV is applied to an NMOS transistor 122 that isconnected in series with a current source 124 and coupled between thesecond input of the Schmitt Trigger comparator 104 and ground. When theovervoltage signal OV has a sufficient magnitude, the transistor 122turns ON causing current to flow through the transistor and currentsource 124 to ground. The transistor 122 and current source 124 togetherform a current limiting circuit 126 that operates during an overvoltagemode of the circuit 100, as will be described in more detail below. Theovervoltage signal OV is further applied to a voltage clamping circuit128 formed by an NMOS transistor 130 and diode-coupled transistor 132coupled between the output of the charge pump 106 and the supply voltageVCC. When the overvoltage signal OV as a sufficient magnitude, thetransistor 130 turns ON allowing current to flow through thediode-coupled transistor 132 and transistor to the supply voltage VCC tothereby clamp the pumped voltage VCCP.

During normal operation of the voltage generation circuit 100, thesupply voltage VCC has a predetermined value and the overvoltagedetector 120 drives the overvoltage signal OV sufficiently low to turnOFF the transistors 122 and 130. Thus, during normal operation thecurrent limiting circuit 126 and clamping circuit 128 do not affectoperation of the voltage generation circuit 100. In operation, theoscillator 102 applies the OSC signal to clock the charge pump 106which, in turn, develops the pumped voltage VCCP. The pumped voltageVCCP is fed back through the diode-coupled transistor 110 and levelshifting circuit 112 to develop the pump feedback voltage VPF. At thispoint, the current flowing through the diode-coupled transistor 116 asdetermined by the current source 118 develops the reference voltageVREF. As long as the pump feedback voltage VPF is less than thereference voltage VREF, the comparator drives the EN signal active,causing the oscillator 102 to clock the charge pump 106.

As the charge pump 106 operates, the pumped voltage VCCP increases to apoint where the pumped voltage fed back through the diode-coupledtransistor 110 and level shifting circuit 112 causes the pump feedbackvoltage VPF to exceed the reference voltage VREF. When the pump feedbackvoltage VPF is greater than the reference voltage VREF, the SchmittTrigger comparator 104 deactivates the EN signal causing the oscillator102 to stop clocking the charge pump 106 which, in turn, turns OFF. Thecharge pump 106 remains OFF until the pumped voltage VCCP dischargesthrough a load resistance 109 and drops to a value causing the pumpfeedback voltage VPF to once again become less than the referencevoltage VREF. When this occurs, the Schmitt Trigger comparator 104 onceagain activates the EN signal causing the oscillator 102 to clock thecharge pump 106, which turns ON to once again begin charging the pumpedoutput voltage VCCP.

When the supply voltage VCC increases, the overvoltage detector 120,current limiting circuit 126, and clamping circuit 128 operate incombination to limit the value of the pumped voltage VCCP. As the supplyvoltage VCC increases, the reference voltage VREF likewise increases,meaning that the pumped voltage VCCP similarly increases to therebyincrease the feedback voltage VPF until it equals the increasedreference voltage. When the supply voltage VCC exceeds a predeterminedvalue, the overvoltage detector 120 activates the overvoltage signal OV,turning ON the transistors 122 and 130. When the transistor 130 turnsON, the pumped voltage VCCP is limited to a value above the supplyvoltage VCC determined by a small voltage drop across the transistor 130plus the voltage drop across the diode-coupled transistor 132.Similarly, the turned ON transistor 122 and current source 124 attemptto sink current in parallel with the current source 118 to increase thevoltage across transistor 116 and thereby limit the increase in thevalue of the reference voltage VREF. Ideally, the reference voltage VREFtracks the supply voltage VCC until the supply voltage exceeds thepredetermined value which activates the overvoltage detector 120. Thismaintains a constant difference between the supply voltage VCC and thepumped voltage VCCP until the supply voltage exceeds the predeterminedvalue. Ideally, once the supply voltage VCC exceeds the predeterminedvalue, the reference voltage VREF is held constant, causing the pumpedfeedback voltage VPF to become greater than the reference voltage, whichcauses the Schmitt Trigger comparator 104 to deactivate the EN signal tothereby deactivate the oscillator 102 and turn OFF the charge pump 106.As will now be explained in more detail, the voltage generation circuit100 does not, however, operate in this ideal manner.

The supply voltage VCC may increase, for example, during burn-in of anintegrated circuit containing the voltage generation circuit 100.Typically, during burn-in the supply voltage VCC is increased to stresscomponents contained within the integrated circuit, as will beunderstood by those skilled in the art. FIG. 2 is a graph illustratingthe values of the pumped voltage VCCP, reference voltage VREF, and theovervoltage signal OV in the voltage generation circuit 100 as thesupply voltage VCC increases. In the example of FIG. 2, the values ofthe supply voltage VCC and pumped voltage VCCP are initially two andthree volts, respectively. At a time T1, the supply voltage VCC beginsto increase and the pumped voltage VCCP and reference voltage VREFsimilarly begin increasing as illustrated. At this point, theovervoltage detector 120 is monitoring the supply voltage VCC but hasnot activated the overvoltage signal OV. Until a time T2, the referencevoltage VREF tracks the supply voltage VCC to maintain a constantdifference between the supply voltage and the pumped voltage VCCP. Atthe time T2, the overvoltage signal OV goes active, turning ON thecurrent limiting circuit 126 and clamping circuit 128. Notwithstandingthe activation of the circuits 126, 128, it is seen that the pumpedvoltage VCCP and the reference voltage VREF continue increasing afterthe time T2. This is true because due to physical limitations, such asheat dissipation and size limitations when forming components of thecurrent source 124, the current limiting circuit 126 cannot sink enoughcurrent to limit the value of the reference voltage VREF as the supplyvoltage VCC increases. As a result, as the supply voltage VCC increasesthe pumped voltage VCCP and reference voltage VREF likewise increase.

In the voltage generation circuit 100, the pumped voltage VCCP maybecome so great as the supply voltage VCC increases that components ofthe integrated circuit containing the voltage generation circuit may bedamaged. For example, the pumped voltage VCCP may exceed the breakdownvoltages of various devices such as MOS transistors formed within theintegrated circuit. Moreover, it should be noted that the clampingcircuit 128 must dissipate what may be significant amounts of power asthe pumped voltage VCCP increases and thus the voltage generationcircuit 100 consumes wasted power and generates unwanted heat during theburn-in process.

There is a need for a voltage generation circuit that reliably limitsthe value of the pumped voltage as the supply voltage increases.

SUMMARY OF THE INVENTION

A method and circuit control the value of generated voltage derived froma supply voltage as the value of the supply voltage varies, such asduring burn-in of an integrated circuit. According to one aspect of thepresent invention, a voltage generation circuit includes a generatorcircuit that receives a supply voltage and has a reference node. Thegenerator circuit develops an output voltage from the supply voltage andthe output voltage has a value that is a function of a reference voltageapplied on the reference node. A coupling circuit is coupled to thereference node and also receives the supply voltage. The couplingcircuit operates in response to a voltage control signal to vary anelectronic coupling of the supply voltage to the reference node whichthereby adjusts the value of the reference voltage. A voltage sensingcircuit receives the reference voltage and develops the voltage controlsignal that is applied to the coupling circuit in response to thereference voltage. The coupling circuit controls coupling of the supplyvoltage to the reference node, adjusting the value of the referencevoltage to control the output voltage of the voltage generation circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram and schematic illustrating aconventional voltage generation circuit.

FIG. 2 is a graph illustrating the effect of an increasing supplyvoltage on an output voltage and several other signals in the voltagegeneration circuit of FIG. 1.

FIG. 3 is a functional block diagram and schematic of a voltagegeneration circuit according to one embodiment of the present invention.

FIG. 4 is a graph illustrating the effect of an increasing supplyvoltage on an output voltage and several other signals in the voltagegeneration circuit of FIG. 3.

FIG. 5 is a functional block diagram of a memory device including thevoltage generation circuit of FIG. 3.

FIG. 6 is a functional block diagram of a computer system including thememory device of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 is a functional block diagram and schematic of a voltagegeneration circuit 300 according to one embodiment of the presentinvention. The voltage generation circuit 300 limits the value of agenerated pumped voltage VCCP as an applied supply voltage VCC increasesso that components within an integrated circuit containing the voltagegeneration circuit are not damaged, as will be explained in more detailbelow. In the following description, certain details are set forth toprovide a sufficient understanding of the invention. However, it will beclear to one skilled in the art that the invention may be practicedwithout these particular details. In other instances, well-knowncircuits, control signals, timing protocols, and software operationshave not been shown in detail in order to avoid unnecessarily obscuringthe invention.

The voltage generation circuit 300 includes a Schmitt Trigger comparator302, oscillator 304, and charge pump circuit 306, which operate in thesame manner as previously described for the corresponding components inthe voltage generation circuit 100 of FIG. 1. For the sake of brevity,these components will not again be described in detail. A more detaileddescription of a charge pump circuit is provided in U.S. Pat. No.6,160,723 to Liu entitled “Charge Pump Circuit Including Level Shiftersfor Threshold Voltage Cancellation and Clock Signals Boosting, andMemory Device Using Same,” and in U.S. patent application Ser. No.09/256,972 to Liu entitled “Method and Circuit for Regulating the OutputVoltage from a Charge Pump Circuit, and Memory Device Using Same” filedon Feb. 24, 1999, both of which are incorporated herein by reference.

A diode-coupled PMOS transistor 308 and current source 310 develop acontrol signal 312. The control signal 312 is applied to a gate of anNMOS transistor 314 that is coupled in series with a current source 316between the supply voltage VCC and ground, and the transistor 314develops a pump feedback voltage VPF in response to the control signal.The pump feedback voltage VPF is applied to one input of the SchmittTrigger comparator 302. A coupling circuit 315 is formed by adiode-coupled PMOS transistor 317, an NMOS transistor 318, and a currentsource 320, which operate in combination to develop a reference voltageVREF in response to an overvoltage control signal OVC, with thereference voltage being applied to a second input of the Schmitt Triggercomparator 302. In response to the OVC signal, the transistor 318adjusts the current through the diode-coupled transistor 317 to controlthe value of the reference voltage VREF.

The diode-coupled transistors 308 and 317 are matched, as are thecurrent sources 310 and 320, which provides common-mode level shiftingof the pump feedback voltage VPF and reference voltage VREF. In theembodiment of FIG. 3, the transistor 314 is a long-channel device thatdevelops a voltage between control signal 312 and voltage VPF of about1.5 volts, which determines the difference between the supply voltageVCC and pumped voltage VCCP, as will be appreciated by those skilled inthe art.

A voltage sensing circuit 322 develops the OVC signal in response to thereference voltage VREF. The voltage sensing circuit 322 includes an NMOStransistor 324, a diode-coupled NMOS transistor 326, and a currentsource 328 that operate in combination to develop a control signal 330in response to the reference voltage VREF. More specifically, thetransistor 324 adjusts the current through the diode-coupled transistor326 in response to the reference voltage VREF to control the value ofthe control signal 330. The control signal 330 is applied to a gate ofan NMOS transistor 332 that is coupled in series with a current source334 between the supply voltage VCC and ground. In response to thecontrol signal 330, the transistor 332 controls the value of the OVCsignal applied to the transistor 318. Thus, the voltage sensing circuit322 forms a feedback circuit that adjusts the value of the OVC signal inresponse to the reference voltage VREF to and thereby control the valueof the reference voltage.

During normal operation of the voltage generation circuit 300, theoscillator 304 clocks the charge pump 306 which, in turn, charges a loadcapacitor 336 to develop the pumped voltage VCCP across the loadcapacitor. In response to the pumped voltage VCCP, the diode-coupledtransistor 308 and current source 310 develop the control signal 312that is applied to the transistor 314 which, in turn, develops the pumpfeedback voltage VPF applied to the Schmitt Trigger comparator 302. Atthe same time, the coupling circuit 315 and voltage sensing circuit 322operate combination to develop the voltage reference VREF that isapplied to the Schmitt Trigger comparator 302. During normal operation,the transistor 332 is turned OFF, causing the OVC signal to go toapproximately the supply voltage VCC and turning ON the transistor 318.In this situation, the value of the reference voltage VREF is determinedby a small voltage drop (less than the threshold voltage of thetransistor 332) across the current source 328 plus the voltage dropacross the diode-coupled transistor 326 plus the threshold voltage ofthe transistor 324.

In the normal operation mode, as long as the pump feedback voltage VPFis less than the reference voltage VREF, the Schmitt Trigger comparator302 enables the oscillator 304 which, in turn, clocks the charge pump306 so that the charge pump continues charging the capacitor 336 toincrease the value of the pumped voltage VCCP. When the pumped voltageVCCP reaches a value causing the pump feedback voltage VPF to becomegreater than the reference voltage VREF, the Schmitt Trigger comparator302 disables the oscillator 304 which, in turn, stops clocking thecharge pump 306. At this point, capacitor 336 begins to dischargethrough a load resistance 337. When the voltage VPF once again becomesless than the reference voltage VREF the Schmitt Trigger comparator 302activates the oscillator 304 to clock the charge pump 306 to charge theload capacitor 336 and increase the pumped voltage VCCP.

The operation of the voltage generation circuit 300 in an overvoltagemode, which occurs when the supply voltage VCC increases such as mayoccur during burn-in of an integrated circuit (not shown) containing thevoltage generation circuit, will now be explained in more detail withreference to FIGS. 3 and 4. FIG. 4 illustrates the values for the pumpedvoltage VCCP, the supply voltage VCC, the overvoltage control signalOVC, the reference voltage VREF, and the control signal 330 duringoperation of the voltage generation circuit 300 in the overvoltage mode.Although not shown in FIG. 4 to simplify the figure, the pumped voltageVCCP has ripple due to the hysteresis of the Schmitt Trigger comparator302. As will now be explained in more detail, during the overvoltagemode the coupling circuit 315 and voltage sensing circuit 322 operate incombination to the to limit the value of the pumped voltage VCCP. Morespecifically, as the supply voltage VCC increases, the voltage on thegate and drain of the diode-coupled transistor 317 increases, and thisincreased voltage is applied through the transistor 318 to increase ofthe reference voltage VREF. In response to the increased referencevoltage VREF, the Schmitt Trigger comparator 302 activates theoscillator 304 which, in turn, causes the charge pump 306 to increasethe pumped voltage VCCP until the pump feedback voltage VPF once againequals the increased reference voltage. In FIG. 4, at a time a T0 thesupply voltage VCC begins increasing and the pumped voltage VCCP andreference voltage VREF likewise increase in response to the increasingsupply voltage.

At a time a T1, the control signal 330 begins increasing from a value ofapproximately zero volts in response to the increasing reference voltageVREF and corresponding increase in current through the transistor 324,diode-coupled transistor 326, and current source 328. At this point,note that the overvoltage control signal OVC also increases andapproximately equals the supply voltage VCC since the transistor 332 isturned OFF. The control signal 330 continues increasing along with theother signals until a time T2, when the magnitude of the control signalequals approximately the threshold voltage of the transistor 332. Inresponse to the control signal 330, the transistor 332 turns ON at thetime T2, causing current to flow through the current source 334 and thetransistor and controlling the value of the overvoltage control signalOVC as illustrated in FIG. 4. When the value of the overvoltage signalOVC is limited at the time T2, the value of the reference voltage VREFis limited to the sum of the threshold voltages of transistors 324, 326,and 332. As a result, the increases in the supply voltage VCC no longerincrease the reference voltage VREF. After the time T2, the pumpedvoltage VCCP no longer increases and is thus limited to prevent damageto components (not shown) in the integrated circuit (not shown)containing the voltage generation circuit 300. Moreover, the powerconsumption of the charge pump 306 does not increase after the time T2notwithstanding further increases in the supply voltage VCC.

FIG. 5 is a block diagram of a memory device 500 including the voltagegeneration circuit 300 of FIG. 3. The voltage generation circuit 300applies the pumped voltage VCCP to a memory-cell array 502 contained inthe memory device 500, and may also apply the pumped voltage to othercomponents in the memory device. In the memory-cell array 502, thepumped voltage VCCP is applied, for example, to word lines (not shown)to access corresponding rows of memory cells (not shown), as will beunderstood by those skilled in the art. The memory device 500 furtherincludes an address decoder 504, a control circuit 506, and read/writecircuitry 508, all of which are conventional and known in the art. Theaddress decoder 504, control circuit 506, and read/write circuitry 508are all coupled to the memory-cell array 502. In addition, the addressdecoder 504 is coupled to an address bus, the control circuit 506 iscoupled to a control bus, and the read/write circuitry 508 is coupled toa data bus.

In operation, external circuitry (not shown) provides address, control,and data signals on the respective busses to the memory device 500.During a read cycle, the external circuitry provides a memory address onthe address bus and control signals on the control bus. In response tothe memory address on the address bus, the address decoder 504 providesa decoded memory address to the memory-cell array 502 while the controlcircuit 506 provides control signals to the memory-cell array inresponse to the control signals on the control bus. The control signalsfrom the control circuit 506 control the memory-cell array 502 toprovide data to the read/write circuitry 508. The read/write circuitry508 then provides this data on the data bus for use by the externalcircuitry. During a write cycle, the external circuitry provides amemory address on the address bus, control signals on the control bus,and data on the data bus. Once again, the address decoder 504 decodesthe memory address on the address bus and provides a decoded address tothe memory-cell array 502. The read/write circuitry 508 provides thedata on the data bus to the memory-cell array 502 and this data isstored in the addressed memory cells in the memory-cell array undercontrol of the control signals from the control circuit 506. The memorydevice 500 may be a dynamic random access memory (DRAM), synchronousDRAM (SDRAM), double-data-rate (DDR) DRAM, packetized memory device suchas an SLDRAM or RAMBUS device, or other type of memory device as well.Moreover, the voltage generation circuit 300 may be placed integratedcircuits other than memory devices.

FIG. 6 is a block diagram of a computer system 600 which uses the memorydevice 500 of FIG. 5. The computer system 600 includes computercircuitry 602 for performing various computing functions, such asexecuting specific software to perform specific calculations or tasks.In addition, the computer system 600 includes one or more input devices604, such as a keyboard or a mouse, coupled to the computer circuitry602 to allow an operator to interface with the computer system.Typically, the computer system 600 also includes one or more outputdevices 606 coupled to the computer circuitry 602, such output devicestypically being a printer or a video terminal. One or more data shortagedevices 608 are also typically coupled to the computer circuitry 602 tostore data or retrieve data from external storage media (not shown).Examples of typical data storage devices 608 include hard and floppydisks, tape cassettes, and compact disk read only memories (CD-ROMs).The computer circuitry 602 is typically coupled to the memory device 500through a control bus, a data bus, and an address bus to provide forwriting data to and reading data from the memory device.

It is to be understood that even though various embodiments andadvantages of the present invention have been set forth in the foregoingdescription, the above disclosure is illustrative only, and changes maybe made in detail, and yet remain within the broad principles of theinvention. For example, some of the components described above may beimplemented using either digital or analog circuitry, or a combinationof both, and also, where appropriate, may be realized through softwareexecuting on suitable processing circuitry. Also, the conductivity typesof the devices, such as NMOS and PMOS transistors, may also be varied asrequired by particular applications, as will be understood by thoseskilled in the art. Therefore, the present invention is to be limitedonly by the appended claims.

1. A method of controlling an output voltage that is derived from asupply voltage, the value of the output voltage being a function of areference voltage that is also derived from the supply voltage, themethod comprising: monitoring the magnitude of the reference voltage; inthe event the magnitude of the reference voltage exceeds a thresholdvoltage, adjusting a relationship between the reference voltage and thesupply voltage in accordance with the magnitude of the reference voltageexceeding the threshold voltage; and generating the output voltage as afunction of the reference voltage having the adjusted relationship whilethe magnitude of the reference voltage exceeds the threshold voltage. 2.The method of claim 1 wherein adjusting the relationship between thereference voltage and the supply voltage comprises increasing animpedance that establishes the relationship between the supply voltageand the reference voltage in proportion to the magnitude of the supplyvoltage.
 3. The method of claim 2 wherein increasing the impedance thatestablishes the relationship between the supply voltage and thereference voltage comprises: generating a control signal from thereference voltage; and in response to the control signal, providing anadjustment signal that is used to adjust a voltage controlled impedancedevice, the adjustment signal having a voltage magnitude related to themagnitude of the reference voltage.
 4. The method of claim 1 whereinmonitoring the magnitude of the reference voltage comprises applying thereference voltage to a voltage sensing circuit having a voltagecontrolled impedance device configured to have an impedance based on themagnitude of the reference voltage.
 5. A method of controlling an outputvoltage that is derived from a supply voltage, the method comprising:generating a reference voltage from the supply voltage having a firstrelationship with the supply voltage; monitoring the value of thereference voltage; adjusting the relationship of the reference voltageto the supply voltage from the first relationship to a secondrelationship in response to the reference voltage exceeding a thresholdvoltage; and generating the output voltage as a function of thereference voltage.
 6. The method of claim 5 wherein adjusting therelationship of the reference voltage to the supply voltage comprisesadjusting a coupling between the supply voltage and a reference node atwhich the reference voltage is provided to control the value of thereference voltage.
 7. The method of claim 6 wherein adjusting thecoupling between the supply voltage and the reference node comprisesadjusting a voltage-controlled device coupled between a source of thesupply voltage and the reference node.
 8. The method of claim 7 whereinadjusting a voltage-controlled device comprises: applying the referencevoltage as a feedback signal; generating a device control signal havinga magnitude that is a function of the feedback signal while thereference voltage exceeds the threshold voltage; and controlling thevoltage-controlled device with the device control signal.
 9. A method ofcontrolling a voltage generation circuit, the voltage generation circuitdeveloping an output voltage from a supply voltage and the outputvoltage having a value that is a function of a reference voltage, themethod comprising: coupling the supply voltage to a reference node todevelop the reference voltage on the reference node, with the amount ofcoupling determining the value of the reference voltage; monitoring thevalue of the reference voltage on the reference node; and adjusting thecoupling of the supply voltage to the reference node responsive to themonitored value of the reference voltage to control the value of thereference voltage; and generating the output voltage in accordance withthe value of the reference voltage.
 10. The method of claim 9 whereincoupling the supply voltage to the reference node to develop thereference voltage comprises adjusting a current flowing through avoltage reference circuit to adjust the value of the reference voltageas a function of the current.
 11. The method of claim 9 wherein thecoupling between the supply voltage and the reference node comprisesadjusting a voltage-controlled device coupled between a source of thesupply voltage and the reference node.
 12. A voltage generation circuit,including, a voltage pump circuit including a reference node and a pumpfeedback node, the voltage pump circuit configured to develop on anoutput node an output voltage having a value that is a function of areference voltage applied on the reference node and a feedback voltageon the feedback node; a feedback circuit coupled between the output nodeand the pump feedback node of the voltage pump circuit, the feedbackcircuit configured to develop the pump feedback voltage in response tothe output voltage; a coupling circuit coupled to the reference node andbeing adapted to receive a supply voltage and a control signal, thecoupling circuit having a variable coupling circuit having a firstsignal terminal coupled to the reference node, a second signal terminalcoupled to the supply voltage, and a control terminal coupled to thecontrol signal, the variable coupling circuit operable in response tothe control signal to control the value of a current supplied from thesupply voltage to control the value of the reference voltage; and avoltage sensing circuit coupled to the reference node to receive thereference voltage and coupled to the coupling circuit, the voltagesensing circuit configured to develop the control signal responsive tothe reference voltage.
 13. The voltage generation circuit of claim 12wherein the voltage pump circuit comprises a voltage pump circuitconfigured to develop as the output voltage a pumped output voltagehaving a value greater than the supply voltage.
 14. The voltagegeneration circuit of claim 12 wherein the voltage sensing circuitcomprises a voltage sensing circuit configured to decrease a value ofthe control signal when the reference voltage increases and increase avalue of the control signal when the reference voltage decreases. 15.The voltage generation circuit of claim 14 wherein the voltage sensingcircuit comprises: a first transistor having a first signal terminalcoupled to the supply voltage and having a second signal terminal and acontrol terminal coupled to the reference node; a level shifting circuithaving a first terminal coupled to the second signal terminal of thefirst transistor and having a second signal terminal, the level shiftingcircuit configured to develop a voltage on the second terminal having avalue that s a function of the voltage on the first terminal; a firstcurrent source coupled between the second terminal of the level shiftingcircuit and a common reference voltage source; a second transistorhaving a control terminal coupled to the second terminal of the levelshifting circuit and having a first signal terminal coupled to thecommon reference voltage source and having a second signal terminal onwhich the voltage control signal is developed; and a second currentsource coupled between the source of the supply voltage and the secondsignal terminal.
 16. The voltage generation circuit of claim 12 whereinthe coupling circuit comprises a coupling circuit configured to increasethe current responsive to a value of the control signal increasing anddecrease the current responsive to the value of the control signaldecreasing.
 17. The voltage generation circuit of claim 16 wherein thecoupling circuit comprises: a level shifting circuit having a firstterminal coupled to a source of the supply voltage and having a secondterminal coupled to the second signal terminal of the variable couplingcircuit, the level shifting circuit configured to develop a voltage on asecond terminal having a value that is a function of the supply voltage;and a current source coupled between the reference node and a commonvoltage reference source.
 18. The voltage generation circuit of claim 12wherein the generator circuit further comprises: a charge pump circuitthat develops a pumped output voltage on an output responsive to a clocksignal; an oscillator circuit coupled to the charge pump, the oscillatorconfigured to develop a clock signal in response to an applied controlsignal being active and not develop the control signal in response tothe applied control signal being inactive; a feedback circuit coupled tothe output of charge pump circuit to receive the pumped output voltage,the feedback circuit configured to develop a pumped voltage having avalue that is a function of the pumped output voltage; and a comparatorcircuit coupled to the oscillator circuit and having a first inputcoupled to the feedback circuit to receive the pump feedback voltage anda second input coupled to receive the reference voltage, the comparatorcircuit configured to apply the active control signal to the oscillatorcircuit when the pump feedback voltage is less than the referencevoltage and apply the inactive control signal to the oscillator when thepump feedback voltage is greater than the reference voltage.
 19. Avoltage generation circuit, including, a generator circuit adapted toreceive a supply voltage and having a reference node, the generatorcircuit configured to develop an output voltage from the supply voltageand the output voltage having a value that is a function of a referencevoltage applied on the reference node; a coupling circuit coupled to thereference node and adapted to receive the supply voltage, the couplingcircuit having a variable coupling circuit having a first signalterminal coupled to the reference node, a second signal terminal coupledto the supply voltage, and a control terminal, the variable couplingcircuit being operable in response to a voltage control signal appliedto the control terminal to vary an electronic coupling of the supplyvoltage to the reference node and thereby adjust the value of thereference voltage; and a voltage sensing circuit coupled to thereference node to receive the reference voltage and coupled to thecoupling circuit, the voltage sensing circuit configured to develop thevoltage control signal responsive to the reference voltage.
 20. Thevoltage generation circuit of claim 19 wherein the generator circuitcomprises a generator circuit configured to develop as the outputvoltage a pumped output voltage having a value greater than the supplyvoltage.
 21. The voltage generation circuit of claim 19 wherein thevoltage sensing circuit comprises a voltage sensing circuit configuredto decrease a value of the voltage control signal when the referencevoltage increases and increase a value of the voltage control signalwhen the reference voltage decreases.
 22. The voltage generation circuitof claim 21 wherein the voltage sensing circuit comprises: a firsttransistor having a first signal terminal coupled to source of thesupply voltage and having a second signal terminal and a controlterminal coupled to the reference node; a level shifting circuit havinga first terminal coupled to the second signal terminal of the firsttransistor and having a second signal terminal, the level shiftingcircuit configured to develop a voltage on the second terminal having avalue that s a function of the voltage on the first terminal; a firstcurrent source coupled between the second terminal of the level shiftingcircuit and a common reference voltage source; a second transistorhaving a control terminal coupled to the second terminal of the levelshifting circuit and having a first signal terminal coupled to thecommon reference voltage source and having a second signal terminal onwhich the voltage control signal is developed; and a second currentsource coupled between the source of the supply voltage and the secondsignal terminal.
 23. The voltage generation circuit of claim 19 whereinthe coupling circuit comprises a coupling circuit configured to reducethe electronic coupling of the supply voltage to the reference noderesponsive to a value of the voltage control signal increasing andincrease the electronic coupling of the supply voltage to the referencenode responsive to the value of the voltage control signal decreasing.24. The voltage generation circuit of claim 23 wherein the couplingcircuit comprises: a level shifting circuit having a first terminalcoupled to a source of the supply voltage and having a second terminalcoupled to the second signal terminal of the variable coupling circuit,the level shifting circuit configured to develop a voltage on a secondterminal having a value that is a function of the supply voltage; and acurrent source coupled between the reference node and a common voltagereference source.
 25. The voltage generation circuit of claim 19 whereinthe generator circuit further comprises: a charge pump circuit thatdevelops a pumped output voltage on an output responsive to a clocksignal; an oscillator circuit coupled to the charge pump, the oscillatorconfigured to develop a clock signal in response to an applied controlsignal being active and not develop the control signal in response tothe applied control signal being inactive; a feedback circuit coupled tothe output of charge pump circuit to receive the pumped output voltage,the feedback circuit configured to develop a pumped voltage having avalue that is a function of the pumped output voltage; and a comparatorcircuit coupled to the oscillator circuit and having a first inputcoupled to the feedback circuit to receive the pumped voltage and asecond input adapted to receive the reference voltage, the comparatorcircuit configured to apply the active control signal to the oscillatorcircuit when the pumped voltage is less than the reference voltage andapply the inactive control signal to the oscillator when the pumpedvoltage is greater than the reference voltage.